1. Field of the Invention
The present invention relates to processes for the preparation of a tetrafluorinated propene. More specifically, this invention concerns processes for the preparation of the hydrofluoroolefin 2,3,3,3-tetrafluoropropene (HFO-1234yf). HFO-1234yf is a refrigerant with low global warming potential.
2. Description of the Related Art
Fluorocarbon based fluids have found widespread use in industry in a number of applications, including as refrigerants, aerosol propellants, blowing agents, heat transfer media, and gaseous dielectrics. Because of the suspected environmental problems associated with the use of some of these fluids, including the relatively high global warming potentials associated therewith, it is desirable to use fluids having the lowest possible greenhouse warming potential in addition to zero ozone depletion potential. Thus there is considerable interest in developing environmentally friendlier materials for the applications mentioned above. Tetrafluoropropenes, having zero ozone depletion and low global warming potential, have been identified as potentially filling this need. However, the toxicity, boiling point, and other physical properties in this class of chemicals vary greatly from isomer to isomer. One tetrafluoropropene having valuable properties is 2,3,3,3-tetrafluoropropene (HFO-1234yf). Thus, there is a need for new manufacturing processes for the production of tetrafluoropropenes and in particular 2,3,3,3-tetrafluoropropene.
U.S. Pat. Nos. 3,819,731 and 3,446,859 disclose the reaction of methyl chloride with either trichloroethylene or tetrachloroethylene in a hot tube reactor to form CCl2═CHCH2Cl or CCl2═CClCH2Cl, respectively. The reaction is believed to occur via a radical process. Tetrafluoroethylene was reacted with methylene produced by photolysis of ketene, resulting in tetrafluorocyclopropane which isomerizes in part to give a tetrafluoropropene believed to be HCF2CH═CF2, and not CF3CF═CH2 (J. Chem. Soc., 1963, 746). Methyl chloride has been pyrolyzed along with CF2HCl at 800° C. to give a product stream that contained about 15% of CF3CF═CH2 (U.S. Pat. No. 2,931,840). HFO-1234yf has also been made by the dehydrofluorination of CF3CHFCH2F with KOH in butyl ether (Chem. Abstr. 1961:349f), and by the reaction of CF3CF2CH2OH with hydrogen (U.S. Pat. No. 4,900,874).
In would be advantageous to have a process for the manufacture of HFO-1234yf that is continuous, vapor phase, and which uses readily available raw materials. As the prior art processes fail in one or more of these desirable features, more advantageous routes are desired, especially those amenable to large scale manufacture.
In general, the process of this invention involves feeding together into a reactor at elevated temperature, a mixture of chlorotrifluoroethylene (CTFE) with a methyl halide to form an intermediate product stream. This step produces HFO-1234yf precursors, such as CF2═CFCH2Cl, that are then fed into a second reactor along with HF, which converts the precursors into HFO-1234yf.